This invention generally relates to placing invisible code markings on articles. Specifically, this invention relates to such a method using aluminum monohydrate as the code marking material.
It is often desirable to mark articles with code numbers which are normally invisible. This can be for esthetic purposes to avoid serial numbers in a location which would create appearance problems. This can also be for security reasons. Television picture tube components are one example of articles which could utilize such invisible code markings. Plate or sheet glass is another example. U.S. Pat. Nos. 2,468,402 and 2,486,566 are examples of prior art transparent or invisible coatings for glass. However, these require an ultraviolet light source to render them visible. I have found that using aluminum monohydrate as the marking material gives a transparent code indicia on an article. This indicia has a greater affinity for indicator solutions, dyes or paints than does the article substrates. Thus, the indicia can be rendered visible rather easily by the spraying of the article with an indicator solution.